For most applications, a single power supply is selected that has a rated output current capacity sufficient to accommodate the maximum expected current demand of the load to which it is connected. However, there is a clear advantage in supplying the current from a plurality of power supply modules, instead of from a single source, particularly if the total load current is relatively high. Perhaps the most significant advantage of a modular power supply system is the substantial improvement in reliability. If any one of the modular power supply sources connected in parallel to the load should fail, the remaining sources can continue to provide the required current, so long as the total rated capacity of the remaining sources is not exceeded. In addition, a single source is generally more expensive than several smaller power supplies having an equivalent total rated capacity, due to the much lower cost of the components used to manufacture the smaller power supplies.
Unfortunately, achieving the ideal modular power supply system is more difficult than it may initially appear. Even nominally identical voltage regulated power supplies have small variations in their output voltage, and as a result, if connected in parallel, do not provide equal current to the load. Connection of the power supply modules in parallel causes their voltage regulation circuits to interact. The source having the highest output voltage tends to provide more current, causing the other sources to respond by further reducing their output voltage. The problem is even worse with power supply modules not provided with voltage regulation. Since the current contributed by each power supply module is not equal, the system operates inefficiently, often with unacceptable losses.
A modular power supply system should also include means for controlling regulation of the parallel connected modular power supplies in the event of a failure of one of the modules. If there are N power supply modules in a system, and K modules fail, it is desirable that the remaining N-K modules continue to share the load current equally. This condition is called fault tolerance of the Kth order.
In a conventional scheme for controlling parallel power supplies, a feedback control circuit is provided for every module. The control circuitry assigned to the Kth module compares the current output from that module to the average current of the system and adjusts the output voltage of the Kth module correspondingly. This approach is referred to as "automatic current sharing."
The above-described prior art system for connecting power supplies in parallel to share current is cumbersome and inadequate for systems in which more than a few modular power supplies are used, because each of the modules must be provided with a control signal comprising the average current provided by all the modules in the system. Thus, it is necessary for each power supply module to communicate with every other module to determine the number of modules that are still functioning and to determine the average current. Further, since the averaging circuitry must be able to adapt as the number of functioning modules in the system changes, it tends to be unduly complex.
In consideration of the above problems, it is an object of the present invention to properly regulate the current supplied by each modular source in a system of parallel connected power supplies, so that each source supplies equal current to the load even though one or more of the sources fails, and to achieve this result without averaging the current supplied by all the modules. This and other objects and advantages of the present invention will be apparent from the attached drawings and the description of the preferred embodiments that follow.